2010ApJ...722..491Z

We study the stellar populations of 1923 elliptical galaxies at z < 0.05 selected from the Sloan Digital Sky Survey as a function of velocity dispersion, σ, and environment. Our sample constitutes among the largest high-fidelity samples of elliptical galaxies with uniform imaging and optical spectroscopy assembled to date. Confirming previous studies, we find that elliptical galaxies dominate at high luminosities (≳L*), and that the highest-σ ellipticals favor high-density environments. We construct average, high signal-to-noise spectra in bins of σ and environment and find the following: (1) lower-σ galaxies have a bluer optical continuum and stronger (but still weak) emission lines; (2) at fixed σ, field ellipticals have a slightly bluer stellar continuum, especially at wavelengths ≲4000 Å, and have stronger (but still weak) emission lines compared with their group counterparts, although this environmental dependence is strongest for low-σ ellipticals and the highest-σ ellipticals are much less affected. Based on Lick indices measured from both the individual and average spectra, we find that (1) at a given σ, elliptical galaxies in groups have systematically weaker Balmer absorption than their field counterparts, although this environmental dependence is most pronounced at low σ; (2) there is no clear environmental dependence of 〈Fe〉, while the α-element absorption indices such as Mg b are only slightly stronger in galaxies belonging to rich groups. An analysis based on simple stellar populations (SSPs) reveals that more massive elliptical galaxies are older, more metal-rich, and more strongly α-enhanced. We also find that (1) the SSP-equivalent ages of galaxies in rich groups are, on average, ∼1 Gyr older than in the field, although once again this effect is strongest at low σ; (2) galaxies in rich groups have slightly lower [Fe/H] and are marginally more strongly α-enhanced; and (3) there is no significant environmental dependence of total metallicity, [Z/H]. Our results are generally consistent with stronger low-level recent star formation in field ellipticals at low σ, similar to recent results based on ultraviolet and infrared observations. We conclude with a brief discussion of our results in the context of recent theoretical models of elliptical galaxy formation.